1. Field of the Invention
The present invention relates to a power detection circuit and, more particularly, to a power detection circuit to be incorporated in a non-contact IC card or a radiofrequency (RF) identification (ID) tag.
2. Description of the Related Art
In recent years, a non-contact IC card and an RFID tag have been widely used. A power detection circuit is requested to cope with a wide range of rise times, which is needed by a power supply incorporated in the non-contact IC card or RFID tag, and to reliably produce a Power On Reset (POR) signal.
A power detection circuit is applied not only to a non-contact IC card and an RFID tag that utilize a non-contact power transmission technology for transmitting power via an antenna but also to various pieces of electronic equipment that control a power supply according to a Power On Reset signal.
The non-contact IC card and RFID tag receive supply of power by radio waves, and internally rectifies the power so as to produce a supply voltage. The positional relationship of a reader writer to the non-contact IC card or RFID tag may vary, and the condition of radio waves varies depending on an ambient environment. The rise time needed by power (a supply voltage) varies depending on a situation but does not remain uniform.
On the other hand, after the power supply of a system is turned on, the system must be initialized in any way. If initialization is imperfect or is not performed, the system may malfunction or may fail.
Therefore, a power detection circuit (Power On Reset (POR) circuit) that produces a POR signal with which a system is initialized after the power supply of the system is turned on has come to play a pivotal role in the non-contact IC card or RFID tag. The present invention is implemented not only in the power detection circuit to be incorporated in the non-contact IC card or RFID tag but also in the power detection circuit to be incorporated in various pieces of electronic equipment.
By the way, for example, Japanese Unexamined Patent Publication (Kokai) No. 03-141415 has disclosed a Power On Reset circuit (power detection circuit) to be incorporated in a MOS-like semiconductor integrated circuit that transmits a Reset signal according to a difference potential between a signal produced as a fraction of a supply voltage using a plurality of MOS transistors and a plurality of resistors and a signal produced as a fraction thereof using two resistors.
Further, for example, Japanese Unexamined Patent Publication (Kokai) No. 02-254811 has disclosed a Reset circuit that resets a logic circuit, which is included in an integrated circuit in which both analog and logic circuits coexist, without using any external part such as an external terminal or capacitor in a state in which the integrated circuit is initialized after the power supply is turned on.
Moreover, for example, Japanese Unexamined Patent Publication (Kokai) No. 09-270686 has disclosed a Power On Reset circuit comprising a charge circuit composed of a resistor and a capacitor; a CMOS inverter that generates a Reset signal until a voltage applied in order to charge the capacitor included in the charge circuit exceeds a predetermined value; a switch that controls a supply voltage to be applied to the charge circuit; an operational voltage designation circuit that designates a voltage with which the switch is activated; a discharge circuit that discharges the charge circuit after the power supply is turned off; and a clamp circuit that retains the switch in an on state after charge is completed. The Power On Reset circuit operates stably irrespective of whether a rise time needed after the power supply is turned on is long or short, and contributes to low power consumption.
In addition, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-068539 has disclosed a simple Power On Reset circuit capable of reliably producing a one-shot pulse irrespective of whether a supply voltage obtained from a power supply after the power supply is turned on needs a long rise time. The Power On Reset circuit includes: a voltage sense means, a shutoff circuit; a supply voltage sense circuit that senses application of a supply voltage when the shutoff circuit is in an on state; a conduction means that conducts a current according to a sense voltage; a capacitor that holds charge, which is received via the conduction means, proportionally to a time constant; a capacitor charge time constant circuit that includes a discharge means; and an output circuit. The Power On Reset circuit applies a voltage, which develops at a second node, as a feedback voltage to the shutoff circuit.
The prior art and its associated problem will be described in detail later with reference to relevant drawings.